Osteosynthesis is achieved by immobilizing separate bone segments and in particular vertebral segments on either side of a failed or damaged disc. When trying to achieve osteosynthesis and specifically fusion between different segments of the spine, one has to provide some type of immobilization. There are various prior art systems which try to achieve this purpose. The different systems involve placement of implants in the form of hooks secured against the bone or pedicle screws threaded into the bone. The implants are then secured to each other by stabilizing rods or plates.
A typical prior art hook implant system, illustrated in FIG. 1, includes a hook implant 10 having a bottom section 10a in the form of a hook shaped finger to be mounted on a bone segment and an upper rod receiving section 10b having a pair of internally threaded posts 10c extending upwardly from a generally unshaped rod receiving cradle or chamber 10d. The posts define a transverse opening 10e for receiving a stabilizing rod 14. A plug 16, e.g., a set screw, having external threads complementary to the internal threads in the posts, completes the system. The set screw, when tightened, clamps the rod and the hook implant together. The term “hook implant”, as used herein, refers to the type of hook 10 illustrated in FIG. 1.
Another type of typical prior art implant for use with a stabilizing rod, commonly referred to as a monoaxial pedicle screw, is illustrated in FIG. 2. The implant 18 consists of a bottom section, in the form of a threaded shaft 18a, joined to an upper rod receiving section 18b formed by a pair of opposed internally threaded posts or branches 18c extending upwardly from a cradle 18d. A threaded plug, e.g., a set screw 16, when threaded into the posts, clamps the implant and rod together. The term monoaxial pedicle screw as used herein refers to the type of implant illustrated in FIG. 2. The monoaxial pedicle screw implant system may include an anti-splay ring 20 (shown in dashed lines) extending around the posts and supporting a flat or curved plate 20a below the set screw so that the plate is forced against the rod by the advancing screw. See, for example, U.S. Pat. Nos. 5,154,719 and 5,385,583.
A prior art implant, initially providing several degrees of freedom between the rod receiving portion of the implant (upper section) and the portion (bottom section) secured to the bone, is commonly referred to as a polyaxial pedicle screw. An implant system employing a polyaxial screw is illustrated in FIGS. 3a and 3b in which the threaded shaft 22a of the bone screw 20 is inserted through a longitudinal opening in a cage or housing 24 with the enlarged head 22b of the screw resting against a reduced cross-sectional area 24c of the housing. The housing, like the upper section of the monoaxial housing, has opposed upstanding internally threaded posts 24c which define a transverse opening or slot 24d for receiving the rod. A set screw 16, when tightened, forces the rod against the head of the screw to clamp the rod, housing and bone screw together. Prior to tightening the set screw the housing may be pivoted relative to the head of the bone screw. See U.S. Pat. No. 6,261,287 and FIGS. 15 and 16. A more advanced polyaxial screw system is shown in FIG. 4 and described in U.S. Pat. No. 6,565,567 in which a pressure washer 26 (initially vertically moveable relative to the housing 28) is placed between the rod and the head 22b of the bone screw 22. The '567 implant system also includes a cap 30 with openings 32a which slide over the posts 28a and a lower lateral connector 32b interposed between the bottom of the set screw 16 and the rod 14.
The term polyaxial pedicle screw as used herein refers to implants of the types shown in FIGS. 3a, 3b and FIG. 4 minus the set screw and cap.
The caps or rings which extend around the internally threaded posts in certain of the above implants serve the purpose of limiting or preventing the outward deflection or splaying of the posts as the set screw is tightened. However, the use of such a cap or ring along with a separate set screw requires two separate steps on the part of the surgeon, i.e., first inserting the cap over the posts and second placing the set screw between posts so that the threads are properly aligned. The step of aligning the set screw with the threaded posts is a problem whether or not a separate cap is used. Since the threads in the posts are interrupted it is difficult, particularly in light of the limited view in the surgical opening for a surgeon to ensure that the set screw is not canted with respect to the opening in the posts, resulting in cross threading and likely a failed installation.
There have been several attempts, by way of patent disclosures, to overcome these problems. See, for example, U.S. Pat. Nos. 5,261,912 (“'912 patent”), 5,667,508 (“'508 patent”), 5,752,957 (“'957 patent”) and 6,440,137 (“'137 patent”). All of such patents teach the use of a unitary locking cap having a male threaded plug or set screw (adapted to be received in the threaded posts) with the cap forming a concentric rim joined to the top of the plug. While this arrangement solves the multiple component problem it presents another problem of raising the vertical profile of the installed implant. The extra space required to accommodate the top of the clamp, which necessarily overlies the top of the implant posts, may result in considerable discomfort to a tall, thin patient particularly if the implant is installed in the thorax region. It is preferable to keep the vertical profile of the implant as short as possible.
In addition, the above unitary cap/screw designs do not entirely solve the screw/threaded posts alignment problem. Such arrangements may tend to allow the cap/screw to pivot or toggle slightly along the openings between the implant posts.
A feature of the present invention is directed towards a solution to such problems by providing an integrally formed cap and set screw with the set screw free to rotate within the cap and the cap/set screw defining means for aligning the set screw with the implant posts. Preferably, the cap defines diametrically opposed openings which slide over the posts with the ends of the openings cooperating with the posts' edges to guide the cap/set screw into alignment with the longitudinal axis of the implant.
In addition to the separate component and alignment problems, conventional implants systems suffer another problem. It is necessary to seat the stabilization rod between the implant posts. This seating step is frequently difficult due to the rigidity of the rod and the vertebra into which the implant has been installed, particularly where the vertebrae is to be moved toward the rod.
To aid the surgeon in accomplishing this task, implants are typically provided with opposed generally cylindrical recesses or slots in the upper section, generally below the rod receiving cradle. A rod persuasion instrument or persuader, in the form of an elongated carriage member, has spaced arms at its distal end which are arranged to slide into the slots or arms provided with inwardly extending flexible fingers arranged to snap into the recesses to releasably fix the distal end relative to the implant. A shaft slidably mounted in the carriage member and actuable from the proximal end (i.e., located exterior of the patient's body) allows the surgeon to push the rod into the receiving chamber in the implant and retain it in that position while the set screw or other clamping means is threaded into the implant posts to lock the rod and implant together. See, for example, U.S. Pat. No. 6,660,006 (“'006 patent”) and U.S. Patent Publication No. US2004/0147936 A1 (“'936 publication). The arms of the '936 device are offset and slide into opposed slots in the upper section below the cradle. This persuader design requires that adjacent implants be at least as far apart as the transverse/offset arms at the distal end of the instrument thereby limiting its use to adjacent implants with the required spacing. In addition, it is difficult to align and slide the offset arms into the slots.
The implant grasping arms at the distal end of the '006 persuader, enclosed within a sleeve or cannula, are flexed outwardly to allow the inwardly projecting fingers to clear the sides of an implant. The arms then snap inwardly to allow the fingers to snap into the recesses at the implant base. The distal end of the cannula is then moved forwardly to retain the fingers within the implant recesses. This arrangement, like the '936 device, also limits the persuader's use on closely spaced adjacent implants. In addition, the securing set screw must be passed through the length of cannula before entering the threaded posts of the implant.
A feature of the present invention, whether of the hook, monoaxial or polyaxial pedicle screw type, overcomes such shortcomings by providing opposed flat recessed sides along the base of the upper section of the implant, aligned with the posts, with the recesses terminating at their upper ends in concave, e.g., outwardly projecting, shoulders.
This arrangement permits the use of a simple persuader which (a) accommodates a close spacing between adjacent implants, (b) eliminates the need for a releasably attachable arm/finger mechanism and (c) is user friendly in terms of placing the distal end of the persuader within the recesses.
Another feature of the present invention, applicable to hook or monoaxial implants, is the design of the cradle to accommodate stabilizing rods of different diameters by providing the cradle with different radii.
With respect to polyaxial screws, a feature of the present invention is to provide a pressure washer with the rod receiving cradle having a slightly smaller radius than the radius of the rod so that the upper edges of the cradle are internally forced outwardly by the rod to create an interference fit. The pressure washer is also preferably made of a material such as a CoCvMo alloy, which has a hardness value about 15% to 22% greater than the hardness value of titanium. The increased hardness of the washer provides a better locking interface between the underside spherical sector of the washer and the screw head thereby requiring less force to secure the rod, polyaxial housing and the pedicle screw together. It is noted that the '567 patent indicates that heat treating titanium may increase the hardness value by about 10% to 20%. However, tests have shown heat treatment actually increases the hardness value by only one or two points on the Rockwell C hardness scale or about 5% or less.